The fuel tanks of aircraft located in wings and horizontal stabilizer (HTP) are fitted with several drainage valves with the purpose of removing the water accumulated in the lower part of those tanks, in such a way that the valves are fitted in the lower skins and also in the manhole covers for the case of a ventilation tank. These covers consist of some removable elements for being able to gain access to the inner space of the wings of the aircraft, wherein the wings constitute the actual fuel tanks.
The part of the valve in contact with the outside of the tank is arranged in the aerodynamic zone in order to prevent problems of parasitic resistance due to possible discontinuities of the surface.
Currently, in order to fit that drainage valve, a recess needs to be created with material that is pulled from the aerodynamic surface of the skin. This recess requires a manual machining with some very small tolerances, which leads to serious problems of quality as far as the finish is concerned, in addition to being a process that is slow and uncontrolled.
So, the said manual recessing for a traditional drainage valve for water is a slow process which does not guarantee the quality required for the later fitting of the valve. Moreover, the cylindrical structure of that recess weakens the lower skin.